This Mixed-Method Action Research study aimed to implement an extended Project-Based Learning professional development series with middle school science teachers. The professional development series took place during the teachers' Communities of Practice, offering them high-quality support, active participation, and content-specific conversations. The study collected data on: (a) changes in teachers' attitudes toward in-district professional development, (b) teachers' perceptions of students' 21st-century skills, and (c) changes in science teacher self-efficacy. Findings from this study add to the growing body of research indicating the need to offer teachers content-specific, supportive, and extended professional development opportunities. Additionally, the findings of this study indicate that teachers using Project-Based Learning in middle school science classrooms perceived an increase in their students' use of 21st-century skills. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]